1. Field of the Invention
The present invention relates an organic EL drive circuit and an organic EL display device using the same organic EL drive circuit and, particularly, the present invention relates to an organic EL display device, which can reduce luminance variation on a display screen of such as a portable telephone set, can achieve high integration density and is suitable for high luminance color display.
2. Description of the Prior Art
It has been known that an organic EL (Electro-Luminescence) display device, which realizes a high luminance display by light generated by itself, is suitable for a display in a small display screen and the organic EL display device has been attracting public attention as the next generation display device to be mounted on a portable telephone set, a DVD player or a PDA (Personal Digital Assistants) such as a portable terminal device, etc.
Known problems of the organic EL display device are that, when it is driven by voltage as in a liquid crystal display device, luminance thereof is substantially voltage dependent and that its sensitivity is color dependent and, therefore, a color display control thereof is difficult.
In view of these problems, an organic EL display device using a current drive circuit was proposed recently. For example, JP H10-112391 A discloses a technique with which the illumination variation problem is solved by employing the current drive system.
FIG. 7 shows an example of a current drive and control circuit of an organic EL display device of such kind, which is currently proposed, and FIG. 8 and FIG. 9 show a current drive circuit thereof.
In FIG. 7, an organic display panel 1 of the organic EL display device for a portable telephone set having 396 (=198xc3x972) terminal pins in a column line and 162 (=81xc3x972) terminal pins in a row line is shown. The organic display panel 1 is constructed with two EL panels 1a and 1b, which are bonded together in center portions thereof.
On the organic display panel 1, two column driver IC""s 2a and 2b and two column driver IC""s 2c and 2d are provided in the upper and lower EL panels 1a and 1b, respectively, and two row driver IC""s 3a and 3b are provided correspondingly to the respective EL panels 1a and 1b. 
In a color display device, each of the column terminal driver IC""s includes 66 terminal pins for each of R, G and B colors, resulting in 198 (66xc3x973) terminal pins forming column output lines. It should be noted that, in FIG. 7, the three different colors are shown without discrimination and, in the following description, the organic display panel 1 has the EL panels 1a and 1b each having 396 (=198xc3x972) terminal pins as the column output lines.
A power source (battery) 4 for driving the organic EL display panel supplies electric power to the column driver IC""s 2a, 2b, 2c and 2d and the row driver IC""s 3a and 3b. The power source voltage thereof is within a range from 12V to 15V and it may be, for example, 15V.
These driver IC""s operate according to a control signal from a controller 5. The column driver IC""s are anode driving drivers for driving anodes of the EL elements and functions as current discharge side to supply currents to the organic EL elements to thereby scan respective output lines as horizontal lines. The row driver IC""s are cathode driving drivers of the organic EL elements and function to sink currents flowing out from the organic EL elements to ground GND to thereby scan respective output lines as vertical lines.
The controller 5 is supplied with electric power from a power source (battery) 7 of 3V and operates under control of a MPU (Micro Processing Unit) 6. The power source 4 may be realized by boosting the voltage of the power source 7 by means of a DC-DC converter.
FIG. 8 is a circuit diagram of one of the column driver IC""s 2a to 2d, which includes 198 column line current driving circuits 8 provided correspondingly to the respective output lines, for current-driving the respective output lines, and a column control circuit 9 provided commonly for the column line current driving circuits 8, for controlling them.
The column line current driving circuit 8 includes a sample and hold circuit 81 for generating a reference drive current, k-time drive current generator circuits 82 each having an input pin 82a supplied with the reference drive current from the sample and hold circuit 81 and amplifying the drive current k times and current mirror output circuits 83 as an output stage for further amplifying the output current of the k-time drive current generator circuits 82 k times. The column control circuit 9 includes a 4-bit D/A converter circuit 91 and a switching control circuit 92.
The sample and hold circuit 81 is a reference current generator circuit (reference power source) driven by the battery 7 of 3V and holds a current data obtained by the D/A converter circuit 91 as a current sample and generates the reference drive current corresponding to an input data value.
Output terminals of the current mirror output circuits 83 as the output stage are connected to respective column pins 84 and driven by the outputs of the k-time drive current generator circuits 82 to generate output currents each being k times the output current of the k-time drive current generator circuits 82, which is k times the reference drive current generated by the sample and hold circuit 81. Thus, the reference current generated by the sample and hold circuit 81 correspondingly to the respective column pins is amplified kxc3x97k times and outputted from the current mirror output circuits 83 to the respective column pins 84.
The generation of the output drive current, which is kxc3x97k times the reference drive current, by the k-time drive current generator circuit 81 and the current mirror output circuit 83 is to reduce the reference drive current to be generated in the sample and hold circuit 81 to the order of xcexc A to thereby reduce power consumption thereof.
The switching control circuit 92 of the column control circuit 9 selectively operates the k-time drive current generator circuits 82 of the column line current drive circuit 8, which are to be horizontally scanned, by sending a switching control signal in response to the control signal from the controller 5. In this case, the data corresponding to a display luminance level in the horizontal scan, which is sent from the controller 5 is preliminarily supplied to the D/A converter circuit 91. The analog signal (analog current value) obtained by the D/A converter circuit 91 is held in the sample and hold circuit 81 as a reference current. The reference current is multiplexed kxc3x97k times by the k-time drive current generator 82 and the current mirror circuit 83, which are selected by the horizontal scan, to produce a drive current and the latter current is outputted to the output pin 84.
FIG. 9 is a circuit diagram of either one of the row drivers 3a and 3b. In FIG. 9, the row driver includes 81 row line current drive circuits 10 provided correspondingly to the 81 output pins for sinking drive currents from the output lines to ground and a switching control circuit 11 commonly connected to the 81 row line drive circuits 10. In FIG. 9, however, only one row line current drive circuit 10 corresponding to a row side pin 81a is shown for simplicity of illustration.
The row line drive circuit 10 is the so-called push-pull output circuit including transistors Tr1 and Tr2, which are driven in the push-pull manner according to a drive signal from the switching control circuit 11. Incidentally, when an output pin to be vertically scanned is selected, the transistor Tr2 on the pull side is turned ON and becomes the current sink side, so that the current, which is outputted from the column side and drives the organic EL element, is sunk to ground GND.
The switching control circuit 11 performs the vertical scan according to the control signal from the controller 5.
In the current-driven organic EL display panel 1 having a large number of pins on the column side, there are problems that a plurality of column driver IC""s are required and that luminance of the display panel is varied every drive IC due to variation of drive currents of the drive IC""s.
As measures against these problems, the drive circuit is formed by using IC""s having substantially equal drive current characteristics. In such case, however, a severe selection of IC""s is necessary, resulting in an increased number of manufacturing steps. In addition, in a color display, characteristics of IC""s for the respective R, G and B becomes a problem and it is difficult to appropriately select IC""s having required characteristics even if the selection of the IC""s is performed inadequately, variation of luminance at a joint between adjacent drive IC""s tends to occur.
When the number of pins of the column terminal drive IC becomes nearly equal to 100 or more (30 pins or more for each of R, G and B), it is difficult to regulate current values of the respective pins on the column side. In addition, in a color display, luminance characteristics of one IC for the respective R, G and B is varied It may be considered in order to regulate the current values that a number of dive current regulation circuits are provided within the IC. However, in such IC, the integration density of the original column current drive circuit is degraded. In order to avoid the degradation of integration density, it may be considered that an external drive current regulation circuit for regulating current from the battery is connected to each IC.
On the other hand, the reduction of size as well as thickness of the organic EL display panel is highly requested and a peripheral mounting area of the panel is limited. Therefore, it is very difficult to mount such external drive current regulation circuits in such limited area. Further, in the above mentioned column line current drive circuit, a number of current mirror circuits corresponding to the number of the pins are required and the number of transistors is increased. Consequently, the lager the number of the output pins results the lower the integration density of IC.
An object of the present invention is to provide an organic EL drive circuit of an organic EL display device, which can reduce the luminance variation in a display screen thereof and have a high integration density.
Another object of the present invention is to provide an organic EL display device, which can reduce the luminance variation in a display screen thereof, have a high integration density and is particularly suitable for use in a high luminance color display.
In order to achieve the above objects, an organic EL drive circuit according to the present invention is featured by comprising a first current mirror circuit provided in a drive stage of a current drive circuit having an output stage for current-driving one of terminals of an organic EL display panel and including n output side transistors connected in current mirror relation to an input side drive transistor, for driving said output stage, where n is an integer equal to or larger than 30, and a drive current regulator circuit for regulating drive current of the input side drive transistor. The input side drive transistor is arranged in a center portion of an arrangement of the n output side transistors and an output current of the output stage is regulated by the drive current regulator circuit.
In the organic EL drive circuit, the drive current regulator circuit is regulated during a fabrication of an IC such that the output current for at least a specific one of the column terminals of the organic EL panel or a current of the output side transistors for the specific terminal becomes a predetermined value.
In a current drive circuit having an input stage for generating a reference current and a current output circuit for current-driving terminals of an organic EL display panel as an output stage, the present inventors constituted, in order to improve the integration density thereof, a drive stage circuit between the input stage and the output stage with a current mirror circuit composed of n output side transistors corresponding to respective pins and one input side drive transistor. Further, in order to remove luminance variation due to column terminal drive IC""s having different characteristics, the present inventors provided a regulator circuit for regulating a reference current (or reference drive current) by selecting resistance values in the column driver IC""s, so that the reference current of each of the column terminal drive IC""s is regulated by trimming the regulator circuit with laser.
According to this construction, an area of the EL display panel is not increased even if the output current regulator circuit is provided on the EL display panel. However, it has been found that luminance variation occurred correspondingly to the column terminal drive IC""s.
The reason for this will be described. When the number of output pins of the column terminal IC becomes as large as 100 (33 or more for each of colors R, G and B), the drive currents are generated by a current mirror circuit having outputs the number of which is 30 or more for one input side, In other words, the output pins are driven in parallel by currents from one reference power source. Therefore, the output currents become different slightly each other, so that there is a difference in output drive current between the first output pin and the last output pin.
In view of this, the present inventors decided that a current regulation is performed such that a current of a last pin of an initial column terminal drive IC becomes equal to a current of a first output pin of a next column terminal drive IC. With such scheme, there may be no luminance variation due to column terminal drive IC""s having different characteristics. However, in the color display, the difference in current between the first pin and the last pin varies between colors. In other words, the luminance characteristics (see FIG. 3) for pin arrangements of colors are different. Therefore, it is difficult to regulate the luminance variation as a whole and the working efficiency is low.
In the color display, the pins for R, G and B are arranged repeatedly sequentially. Therefore, the relation between the last pin of a certain column terminal drive IC and the first pin of a next column terminal drive IC corresponds to the relation between the third pin from the last pin of n pins and the first pin of the next column terminal drive IC for color G, to the relation between the second pin from the last pin and the second pin of the next column terminal drive IC for R and to the relation between the last pin and the third pin of the next column terminal drive IC for color B.
The luminance variation in the case where output pins are driven in parallel by currents from one reference power source will be described in more detail. The luminance variation due to difference of the drive current for the column terminal drive IC""s may not be serious when the number of pins of R, G and B are about 10, respectively. However, it has been found that, when the number of pins of the column terminal drive IC for each of R, G and B colors becomes 33, the luminance variation becomes serious. It has been found that such luminance variation can not be reduced even if the number of 33 pins for each of R, G and B is reduced by about 10%, respectively. The output currents of current mirror output circuits for supplying drive currents to the column output pins of the column terminal drive IC""s for colors R, G and B were measured and output pin vs. output current characteristics shown in FIG. 3 was obtained for the respective colors. In FIG. 3, abscissa indicates positions of the output column side output pins and ordinates indicates an output current Io. In order to solve the problem of difference in characteristics curves of the column terminal drive IC""s for colors G, R and B, it has been considered as mentioned previously to provide the reference current source and the current regulator circuit for each of R, G and B and to regulate the currents by laser trimming. However, as shown in FIG. 3, the difference between the characteristics curves for R, G and B are too large to restrict the luminance variation. The present inventors investigated the reason for such large difference in the R, G and B characteristics curves and have found that the large difference is due to the drive stage of the current mirror circuit including one input side transistor and 33 output side transistors. That is, when, in order to reduce the power consumption, the drive current to be generated in the output side transistors of the current mirror circuit is set to the order of xcexcA, the characteristics curves for these colors become difficult.
That is, the characteristics curves are largely influenced by the wiring resistance due to miniaturization of the wiring line for generating such small currents in the drive circuits, the degradation of the base-emitter characteristics due to miniaturization of the transistors of the drive circuits and the layout of the R, G and B drive circuits.
In the layout of the driver circuits for colors R, G and B, the driver circuits for colors G and B are usually arranged on both sides of the driver circuit for color R. Therefore, the current drive lines for R, G and B are different. Further, it becomes difficult to widen the drive wiring line with increase of the number of the output pins, so that the width thereof is usually several tens microns and the wiring resistance can not be reduced sufficiently. In addition, the wiring line is formed of such as aluminum whose electric conductivity is relatively low. That is, the resistance of the unit length of wiring becomes relatively large. Although the integration density of IC is improved by reducing the width of the wiring line, the output pin vs. output current characteristics thereof is degraded. Further, when the width of the power supply line, which is common for the output transistors of the drive circuit, is reduced, the drive current vs. pin characteristics is degraded.
In order to solve these problems, the integration density is improved by providing, as the drive stage for the output stage of the current drive circuit of the column lines of the organic EL display panel, the current mirror circuit having a output side transistors for each input side drive transistor, where n is an integer equal to or larger than 30. Further, the input side drive transistor is arranged in substantially the center of the arrangement of the n output side drive transistors, so that the drive current of the first pin becomes substantially equal to the drive current of the last pin for the R, G and B, which are arranged in substantially symmetrical positions about the center. With these schemes, the knoll type drive current characteristics is obtained. As a result, the luminance characteristics for the pin arrangement becomes similar to the drive current characteristics. Further, the drive current for at least a specific pin of the pins of each of colors is regulated to a predetermined value by the drive current regulator circuit.
With these measures against the problems, the output pin vs. output current characteristics of the current drive circuit for each color according to the present invention becomes substantially symmetrical knoll type curve such as shown in FIG. 4 and the position in the height direction of the knoll type characteristics curve can be regulated by the drive current regulator circuit. Therefore, it becomes possible to make the output pin vs. output current characteristics for colors R, G and B substantially equal. Further, the knoll type characteristics curve of the drive current can moderate the luminance variation in the pin arranging direction.
As a result, the current drive circuit according to the present invention can reduce the variation of drive current between the column terminal drive IC""s and can suppress the variation of luminance between a certain column terminal drive IC, that is, the anode drive IC of the organic EL, and a next column terminal drive IC (anode drive IC). Therefore, it becomes possible to reduce the luminance variation on a whole display screen to thereby provide an organic EL display device having an improved integration density and a capability of high luminance color display.
According to the present invention, it is possible to make the luminance characteristics curves for color R, G and B even in one column terminal drive IC (anode drive IC of the organic EL), so that it is possible to realize the column terminal drive IC of the organic EL display device suitable for high luminance color display.
Incidentally, in the following description for each of R, G and B, the drive pins of the column direction in the description for each of R, G and B are numbered from 1st to 33rd and, in the description for R, G and B as a whole column terminal drive IC, the pins are numbered from 1st to 99th without distinction between R, G and B.